In recent years, as means for shaping steel sheet for auto parts using high strength steel sheet, hot press forming has increasingly been employed. Hot press forming shapes the steel sheet at a high temperature to thereby form it at a stage of a low deformation resistance and then rapidly cools it to quench harden it. With hot press forming, it is possible to press-form parts which are high in strength and are high in shape precision without causing deformation or other shaping problems after shaping.
Specifically, with the hot press forming method, first, steel sheet which has been heated in advance by a heating furnace to a predetermined temperature is supplied to a press die. After this, in a state placed on the bottom die (die) or in a state lifted from the bottom die by lifters or other fixtures built in the bottom die, a top die (punch) is descended to the bottom die limit. Next, the steel sheet is cooled for a certain time (usually 10 seconds to 15 seconds) to cool the steel sheet to a desired temperature. Further, after the cooling finishes, the shaped steel sheet is taken out from the die, then a new steel sheet which has been heated to a predetermined temperature is supplied to the press die. The steel sheet is quenched, tempered, and otherwise heat treated in the cooling process. Therefore, in hot press forming, freely controlling the cooling rate from the viewpoint of the heat treatment characteristics of the steel sheet, obtaining a uniform cooling rate at the steel sheet as a whole from the viewpoint of stability of quality, and shortening the time required for the cooling process after shaping the steel sheet from the viewpoint of productivity, are important.
As means for shortening the cooling time of the shaped steel sheet, it has been proposed to not make the die directly rob heat from the steel sheet, but to feed another medium, for example, water, to the surface of the steel sheet (for example, PLT 1). In particular, in the hot press forming apparatus which is described in PLT 1, the inside surface of the die is provided with a plurality of independent projections of certain heights and channels for water which are communicated with plurality of locations at the inside surface of the die are provided inside the die. Due to this, it is possible to run coolant through the channels inside of the die in the clearances, which are formed by the projections, between the inside surface of the die and the steel sheet. For this reason, it is possible to cool the metal sheet in a short time and raise the productivity of the hot press forming operation. Further, this quenching by rapid cooling enables the steel sheet to be raised in hardness and the strength of the shaped part to be greatly improved.
Further, as means for shortening the time which is required for the cooling process after shaping the steel sheet, it has been proposed to arrange a storage container storing a coolant as close to the steel sheet as possible (for example, PLT 2). In particular, the die which is described in PLT 2 is provided with a storage container which stores a coolant, a plurality of feed holes which feed coolant which is stored in the storage container to the steel sheet, and a coolant feed control device which is provided between the storage container and the feed holes. By having a storage container of coolant arranged inside the die in this way, it is possible to shorten the distance between the storage location of the coolant and feed locations of the coolant. Due to this, it becomes possible to immediately feed coolant to the steel sheet after the control device is sent a coolant feed instruction, and therefore the time from press forming the steel sheet to the end of the cooling process can be shortened.